The present invention relates to a video signal transmission equipment for transmitting a video signal of a vestigial sideband modulated wave in a wire system or a wireless system.
A conventional video signal transmission equipment is described in Japanese Laid-open Patent Application No. Hei 2-202726, and this equipment will be described with reference to FIG. 6 to FIG. 9.
FIG. 6 is a block diagram showing the configuration of the conventional video signal transmission equipment. In the figure, the video signal transmission equipment comprises an input device 31 to which a video signal is input, and the output terminal of the input device 31 is connected to on input terminal of a VHF modulation circuit 33. To the other input terminal of the VHF modulation circuit 33, the output terminal of a carrier oscillation circuit 32 is connected and a carrier wave of a frequency fp is input. The output terminal of the VHF modulation circuit 33 is connected to the input terminal of a VHF surface acoustic wave filter 34 for vestigial sideband filtering (hereinafter, abbreviated as VSBF 34). The output terminal of the VSBF 34 is connected to the input terminal of an amplifying circuit 35, and a video output signal is obtained from the output terminal of the amplifying circuit 35. The VSBF 34 is a band pass filter having a frequency characteristic shown in FIG. 8.
The operation of the video signal transmission equipment configured above will be described hereafter.
The video input signal to the input device 31 is set to a predetermined level, and is output as an output signal. The output signal of the input device 31 is supplied to the VHF modulation circuit 33, and the carrier signal input from the carrier oscillation circuit 32 to the VHF modulation circuit 33 is amplitude-modulated. The frequency spectrum of the output signal of the VHF modulation circuit 33 is shown-in FIG. 7. Then, the output signal is applied to the VSBF 34. The frequency components of the upper sideband and a part of the lower sideband of the output signal pass through the VSBF 34, so that a modulated wave is obtained which has a vestigial sideband shown in FIG. 9. The output signal of the VSBF 34 is power-amplified by the amplifying circuit 35, and transmitted as the video output signal.
The above-described conventional video signal transmission equipment has problems shown below. Since the oscillation frequency fp of the carrier oscillation circuit 32 is fixed and the frequency characteristic of the VSBF 34 is also fixed and unchangeable, only a video signal for one transmission channel can be obtained. In order to change the transmission channel, it is necessary to change the oscillation frequency fp of the carrier oscillation circuit 32 and change the frequency characteristic of the VSBF 34. To handle plural transmission channels, it is necessary to prepare as many VSBFs 34 as the number of transmission channels. It has been necessary to change the circuit or replace a part in order to change the oscillation frequency fp and change the frequency characteristic of the VSBF 34.
A first object of the present invention is to provide a video signal transmission equipment capable of transmitting a video signal on a specified channel of plural channels without changing the circuit or replacing a part.
A second object of the present invention is, in addition to the above-mentioned first object, to provide a video signal transmission equipment configured so that a video signal is not transmitted on a channel other than the specified channel.
A third object of the present invention is, in addition to the above-mentioned first object, to provide a video signal transmission equipment capable of transmitting a video signal on a predetermined auxiliary channel when the video signal of the specified channel cannot be transmitted.
To achieve the above-mentioned first object, a video signal transmission equipment in a first aspect of the present invention comprises: a first oscillator for outputting a sinusoidal signal of a frequency being higher than a carrier frequency of a video signal which is transmitted from the video signal transmission equipment; an amplitude-modulator for amplitude-modulating an output signal of the first oscillator by a video baseband signal; a first filter for filtering an output signal of the amplitude-modulator, and having a particular frequency characteristic; a second oscillator oscillating at a frequency range which is higher than an oscillation frequency of the first oscillator; control means for controlling an oscillation frequency of the second oscillator; a frequency converter for multiplying an output signal of the second oscillator and an output signal of the first filter; and a second filter for low-pass-filtering an output signal of the frequency converter.
According to the video signal transmission equipment in the first aspect, an effect is obtained that the video signal can be transmitted on a specified channel of plural channels without changing the circuit or replacing a part.
To achieve the second object, a video signal transmission equipment in a second aspect of the present invention comprises in the above-mentioned video signal transmission equipment: detecting means for detecting a condition of synchronism between the output signal of the second oscillation means and a reference clock signal; and inhibiting means for inhibiting transmission of a video signal from the second filter when the second oscillation means is unlocked.
According to the video signal transmission equipment in the second aspect, in addition of the effect of the first video signal transmission equipment, an effect is obtained that a video signal which becomes an interference signal for other channels is never transmitted when the second oscillation means is unlocked.
To achieve the third object, in video signal transmission equipment in a third aspect of the present invention, in the video signal transmission equipment In the first aspect, the oscillation frequency of the first oscillator is made variable, and the following are further provided: detecting means for detecting a condition of lock between the output signal of said second oscillator and a reference clock signal; and switching device for causing the output signal of said first filter to be directly transmitted on a predetermined channel when the output signal of said second oscillator and the reference clock signal are unlocked.
According to the video signal transmission equipment in the third aspect, in addition to the effect of the video signal transmission equipment in the first aspect, an effect is obtained that the video signal is transmitted on an auxiliary channel so that the video signal does not become an interference signal for other channels even when the output signal of the second oscillation means and the reference clock signal are unlocked.